I have a 97 E420 with the notorious surging idle problem which I’ve been trying to fix on and off for a while now. The car has 120k miles.

I recently noticed that the hose coming from the bottom portion of the MAF sensor to the upper left hand side of the cylinder wall had holes in more than one place and was practically melted away (hose is the consistency of chewing gum). Looked this up on the web and found it to be the air distribution hose. I also noticed small puffs of white smoke coming from the holes. The engine typically runs at 100C when at a standstill and falls to 80 when moving. I didn’t think that this seemed excessively hot. Could the sensor be wrong? I will obviously be replacing this hose but I’m wondering if anyone has any details regarding its purpose and if those holes could be contributing to the idle surge problem. I’m a bit concerned that there are deeper problems within my engine and if I replace the hose it will just melt away again.

I also tried disconnecting the electric cable attached to the coolant temperature sensor in front portion of the engine. This caused the idle to pick up to about 7 and seemingly cured the idle surge! However, I’m not sure if the change is due to a safe mode that the computer enters. This could potentially explain both the idle surge and the high temperatures. Is this something I should consider replacing? If so is it a DIY job?

Here are some things I noticed about the idle surge:
- The car idles just above 5 then it starts to drop off to around 4 or below before the computer catches it and revs the rpm up past five where it starts to drop off and the cycle repeats.
- The problem is much worse (more pronounced and more frequent) in warmer weather.
- only occurs when in drive and the car is not moving. Fine at higher rpms
- CEL light keeps coming up with the low gas light flashing. It always comes up right when the idle surges. When I pull the codes I get P0455. Tried replacing the gas cap but still get the CEL. Also replaced: plugs, fuel filter, air filter and the MAF unit as local mechanics had suggested. No luck.

Could the hot engine / broken air distribution hose be causing the rough idle? It seems to me that there is something else wrong with this engine that I am overlooking. Any help or feedback would be greatly appreciated!

Thanks,
Jon

on this forum a couple of weeks ago about a user with a surging M104 engine.
He had bad hoses replaced and car ran fine. Seems to me like the bad hose on your car would definitely mess up vacuum system which must work properly to avoid P0455 error.

Start here: OK,,, I give up